[flang] Silence a bogus warning

A recent fix (llvm#66232)
to interpret a hitherto unknown name whose first appearance is as
the target of a procedure pointer initializer as an external
procedure has led to some inapproprite warning messages if the
name is later defined as an external subroutine ("X was already
declared as an external").

Ensure that the EXTERNAL attribute is correctly noted as being
implicit, and that it's okay that neither the Subroutine nor Function
flag has yet been set for the implicit external.

Author: klausler

flang/lib/Semantics/resolve-names.cpp

@@ -4191,18 +4191,23 @@ bool SubprogramVisitor::HandlePreviousCalls(
     // ENTRY's name.  We have to replace that symbol in situ to avoid the
     // obligation to rewrite symbol pointers in the parse tree.
     if (!symbol.test(subpFlag)) {
+      auto other{subpFlag == Symbol::Flag::Subroutine
+              ? Symbol::Flag::Function
+              : Symbol::Flag::Subroutine};
       // External statements issue an explicit EXTERNAL attribute.
       if (symbol.attrs().test(Attr::EXTERNAL) &&
           !symbol.implicitAttrs().test(Attr::EXTERNAL)) {
         // Warn if external statement previously declared.
         Say(name,
             "EXTERNAL attribute was already specified on '%s'"_warn_en_US);
-      } else {
+      } else if (symbol.test(other)) {
         Say2(name,
             subpFlag == Symbol::Flag::Function
                 ? "'%s' was previously called as a subroutine"_err_en_US
                 : "'%s' was previously called as a function"_err_en_US,
             symbol, "Previous call of '%s'"_en_US);
+      } else {
+        symbol.set(subpFlag);
       }
     }
     EntityDetails entity;
@@ -8163,6 +8168,7 @@ bool ResolveNamesVisitor::Pre(const parser::PointerAssignmentStmt &x) {
       // Unknown target of procedure pointer must be an external procedure
       Symbol &symbol{MakeSymbol(
           context().globalScope(), name->source, Attrs{Attr::EXTERNAL})};
+      symbol.implicitAttrs().set(Attr::EXTERNAL);
       Resolve(*name, symbol);
       ConvertToProcEntity(symbol);
       return false;